Annual depreciation expense then is computed by applying this accelerated depreciation rate to the undepreciated cost ( current book value ) of the asset .

年折旧费就用这个加速折旧率 乘以 待 折旧成本（ 即当时的账面净值）得出。

As long as each year 's depreciation expense is equal to only a portion of the undepreciated cost of the asset the asset will never be entirely written off .

只要每1年的折旧费都仅等于资产的 待 折旧成本的一部分，该项资产就永远不会被全额冲销。